Moldings of poly(alkylene terephthalate) resins such as poly(butylene terephthalate) (PBT) and poly(ethylene terephthalate) (PET) and polyamide resins such as nylon 6 and nylon 66 are insufficient in their impact strength, and many attempts have been made to improve the impact strength. Many agents have been proposed to improve the impact strength which are to be added to resins and subjected to melt-blending. Among the agents proposed so far, a core-shell polymer comprising a core made of rubbery polymer and a shell, around the core, made of glassy polymer is an excellent agent for improvement of impact strength as far as attaining high impact strength, because its reproducibility is high, since its dispersion into resin is hardly affected by the conditions of melt-blending.
A multiphase polymer of which the outermost phase has epoxy groups has been proposed in Japanese Patent Application Laid-Open No. 74652/77 as a core-shell polymer for improvement of the impact strength of poly(alkylene terephthalate) resins. However it is not practicable because it can improve the impact strength only slightly after melt-blending with a poly(alkylene terephthalate) resin, and practically a considerable proportion of the epoxy groups included therein have been lost during the processes of polymerization and drying. Japanese Patent Application Laid-Open No. 150466/77 has proposed compositions of core-shell polymers having no epoxy groups with poly(alkylene terephthalate) resins; some of these compositions of which have shown high notched Izod impact strength (thickness: 1/8 inch) at 25.degree. C. or higher. However even these compositions with high impact strength show a change from ductile fracture to brittle fracture (ductile-brittle transition) i.e., at about 20.degree. C., below which they show only a low notched Izod impact strength.
As core-shell polymers which can improve the impact strength of polyamide resins, Japanese Patent Application Laid-Open No. 6284/72 has proposed compositions of core-shell polymers of which the outermost phase contains carboxyl groups, with polyamide resins; some of these compositions of which have shown high notched Izod impact strength (thickness: 1/8 inch) above 23.degree. C. However even these compositions with high impact strength exhibited ductile-brittle transitions at about 15.degree. C., below that which they exhibited only low Izod impact strength.
On the other hand, engineering plastics, such as poly(alkylene terephthalate) resins, polyamide resins, modified poly(phenylene oxide) resins, polyacetal resins, and polycarbonate resins, require sufficiently high impact strength in the range of from room temperature to -30.degree. C. when they are to be used for the outer-parts of cars. However as described above, the prior art had a problem in that poly(alkylene terephthalate) resin compositions and polyamide resin compositions produced by melt-blending impact modifiers of the core-shell polymer show ductile-brittle transitions at room temperature (15.degree. to 25.degree. C.) in the notched Izod impact test (thickness: 1/8 inch), having only low Izod impact strength below room temperature.